To lose weight

To lose weight, you should eat little less calories than your body needs per day.
To lose 1 kg of your weight, you should create 7700-calorie deficit.
To lose 1 kg per week, you should create approximately 1000-calorie deficit every day.

For example, if your daily calorie need is 2800 calories (calorie burning from your daily activities and exercise), then you should take 1800 calories from your diet every day. In this case, you are creating calorie deficit of 1000.

HOW MANY CALORIES YOU SHOULD EAT PER DAY TO LOSE WEIGHT?
Follow these steps:
Step-1: Calculate your daily calorie burning from http://www.calculator.net/calorie-calculator.html

Step-2: Eat 1000 less calories than your ‘daily calorie burning’ to lose 1 kg per week.

Here’s a simple example to make you understand better.
I am using following data for calculation:
Gender: female
Current Weight: 170 pounds
Height: 5’3”
Age: 30
Lifestyle: Very Active (Cardio 6 Days + Strength Training 5 Days)

Result (I used this calculator and calculated the following):

She needs 2,519 Calories/day to maintain her weight.
She needs 2,019 Calories/day to lose 1 lb per week.
She needs 1,519 Calories/day to lose 1 kg per week.